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Summary Theory has shown that time lags in the regulation of symbiotic nitrogen (N) fixation (SNF) can be important to the competitive dynamics and ecosystem consequences of N‐fixing trees, but measurements of these time lags are lacking.Here, we used a novel method to measure SNF in seedlings of four N‐fixing tree species that represent tropical and temperate origins and actinorhizal and rhizobial symbiotic associations, each grown under warm and cold temperature regimes. We added N to previously N‐poor pots to induce downregulation and flushed N out of previously N‐rich pots to induce upregulation.It took 31–51 d for SNF to decline by 95%, with faster downregulation in temperate species and at warm temperatures. Upregulation by 95% took 108–138 d in total, including 21–57 d after SNF was first detectable. SNF started earlier in rhizobial symbioses, but increased faster once it started in actinorhizal symbioses.These results suggest that time lags in regulating SNF represent a significant constraint on facultative SNF and can lead to large losses of available N from ecosystems, providing a resolution to the paradox of sustained N richness.